"Der Fernmelde=Ingenieur", vol. 42, No. 9, September 1987, especially item 3.4 on pages 8 and 9 discloses method and apparatus for switching messages in the form of uniform length packets. European patent Publication No. EP-A2-0 125 744 "Closed Loop Telecommunication System" describes a ring system in which a complete frame containing a plurality of packets circulates at all times, using shift registers or other memories.
The increasing diversity of telecommunication services necessitates a highly flexible switching system. For this purpose, fast packet-switching systems, in which the data are handled in packet form, are being considered in particular. In such systems, delays, delay jitter, and packet losses cannot be excluded. To this, however, the individual telecommunication services are differently susceptible. For example, data services which detect the loss of a packet and can call for this packet anew are far less susceptible to packet losses than voice or moving-image services, which must evaluate the incoming information immediately (electroacoustic or electrooptical conversion) and in which losses and delay jitter cause disturbances.
It is known to use packets of uniform length in a packet transmission system, to combine these packets into periodically recurring frames, to assign fixed positions within the frames to messages to be treated with priority, and to switch these messages to be treated with priority using synchronous time-division multiplexing as is commonly done in circuit-switching systems, while the other packets are packet-switched in the usual manner.
For the messages to be treated with priority, a synchronous transfer mode (STM) is obtained. The individual packets are called "STM cells". For the other messages, an asynchronous transfer mode (ATM) is obtained, in which the individual packets are called "ATM cells". In connection with the synchronous transfer mode, terms such as "circuit switching" (CS) and "CS packets" are used; in connection with the asynchronous transfer mode, terms such as "packet switching" (PS), "PS packets", "asynchronous time division" (ATD), and "fast packet switching" are used. Combinations are called "hybrids".
However, a packet inevitably contains more bits than a time slot in conventional time-division multiplex systems. Frames containing about 70 40-octet packets, i.e., about 40.times.70.times.8 bits=22,400 bits, are currently under discussion. In conventional time-division multiplex systems, frames with 32 16-bit time slots, i.e., 512 bits, are commonly used. Since in a synchronous time-division multiplex switching system one complete frame per input must be temporarily stored in each time switch, the amount of storage required increases considerably. Moreover, delays of the order of one frame occur. If, as is to be expected, the frame repetition rate is the same in both cases (8 kHz), the memories will have to be not only considerably larger but also correspondingly faster. Speed reduction by parallel instead of serial processing is possible only to a very limited extend if at all.